This disclosure relates generally to the fuel cell field, and more particularly to fuel cell systems and methods for operating the fuel cell systems.
Fuel cells are electro-chemical devices which can convert chemical energy from a fuel into electrical energy through an electro-chemical reaction of the fuel, such as hydrogen, with an oxidizer, such as oxygen contained in the atmospheric air. Fuel cell systems are being widely developed as an energy supply system because fuel cells are environmentally superior and highly efficient. To improve system efficiency and fuel utilization and reduce external water usage, the fuel cell system usually includes an anode recirculation loop. As single fuel cell can only generate 1V voltage, therefore, a plurality of fuel cells are usually stacked together (usually referred to as a fuel cell stack) to get desired voltage.
A fuel utilization (UF) is a critical variable in the fuel cell systems. The fuel utilization indicates the ratio of consumed equivalent hydrogen to the net available equivalent hydrogen in an anode of the fuel cell. The equivalent hydrogen of a mixed gas refers to the sum of molar flowrate of each species multiplied by the number of hydrogen it can produce.
The fuel utilization may affect efficiency of the fuel cell system. High fuel utilization may enhance fuel efficiency of the fuel cell system, so the high fuel utilization may imply high fuel efficiency of the fuel cell system. However, increasing fuel utilization may also increase the risk of fuel starvation, as insufficient fuel, such as hydrogen, is present in the anode of the fuel cell, which may cause irreversible damages of the fuel cell due to anode oxidation.
Thus, tight control of the fuel utilization may play an important role in preventing fuel starvation and improving system efficiency. However, it is difficult to measure the fuel utilization of fuel cell stack, due to the difficulty to measure the recycling flowrate, methane (CH4), Carbon Monoxide (CO), hydrogen (H2), Carbon Dioxide (CO2) and water vapor (H2O) in real time. Although the fuel utilization of the fuel cell system (which is solely defined by electrical current drawn from the fuel cell stack and fuel flow rate provided into the anode recirculation loop) has been used for fuel cell operation and control, the operating boundaries for fuel utilization of the fuel cell system are usually determined by trial and error, due to no explicit relationship between fuel utilization of the fuel cell system and fuel utilization of the fuel cell stack.
Therefore, there is a need for a system to obtain the fuel utilization of the fuel cell stack in real time by using easily measured variable. There is a further need for a system to predict and update the operating boundaries for fuel utilization of fuel cell system in real time. There is still a further need to operate and control the fuel cell system to simultaneously prevent fuel starvation and carbon deposition.